Uranium is an important emerging toxicant. Exposure to natural uranium has been linked to lung and kidney diseases in uranium miners, and exposure to depleted uranium has been weakly linked to lung cancer in soldiers. Human epidemiology has focused almost exclusively on uranium's radiological toxicity rather than its chemical toxicity. Furthermore, the photoactivation of uranium by UV light is a well-used laboratory tool that has not been considered as a mode of action for uranium toxicity in the skin. The overall goal of this pilot project is to evaluate the following hypothesis: If uranium is photoactivated by UVB light to form uranium(V) and reactive oxygen, then co-exposure to UVB light and uranium may be synergistically mutagenic, through a mode of action that is different than either agent alone. This hypothesis will be tested by measuring the ability of uranium and UVB light to cause mutations at the hprt locus of human keratinocyte HaCat cells. The localization of uranium in the nuclei of exposed cells will also be probed by electron microscopy combined with Energy Dispersive X-ray Spectroscopy. This project will provide data that may challenge the current paradigm that uranium is predominantly a radiological mutagen. Results from this work may provide evidence for chemical modes of action, specifically, that the photoactivation of uranium(VI) can cause mutations that are different than those caused by either uranium or UVB light alone. The proposed work may also provide data to expand the focus of uranium toxicity from lung and kidney to include skin as a target organ for carcinogenesis, and may drive the generation of new hypotheses for future epidemiological studies. PUBLIC HEALTH RELEVANCE: The purpose of this work is to determine how the heavy metal uranium damages DNA. We will measure the ability of uranium to be activated by UV light, and will measure the DNA mutations that occur from this process. Data may support a mechanism for uranium to be involved in skin cancer.